Narrowband communication links, such as those incorporated in line-of-sight (including satellite) data transmission systems, commonly employ a continuous phase modulation digital signalling scheme in order to achieve efficient use of the available data channel and to take advantage of the constant envelope characteristics of the modulation format and thereby avoid unwanted degradation of the transmitted signal due to the use of non-linear amplifier components (e.g. travelling wave tubes). A diagrammatic illustration of a digital data communication system employing continuous phase modulation (such as that described in U.S. Pat. No. 4,071,829 entitled "Coherent Phase Detector Using a Frequency Discriminator" and assigned to the assignee of the present application) is shown in FIG. 1 as including an input link 11 over which (M-ary) digital data is coupled to a modulator for transmission. As described in the above-referenced patent the modulator contains a baseband aperture filter 12 for applying a finitely shaped (i.e. squarewave) input signal (of the form kd.PHI..sub.m /dt to a voltage controlled oscillator 13 which produces a continuous phase outputsignal of the form cos (.OMEGA..sub.o t+.PHI..sub.m (t)). The output of voltage controlled oscillator 13 is coupled to an RF amplifier (TWT) 14 for transmission over a communication link (e.g. satellite channel) 15 to a receiver site.
At the receiver the incoming signal, which has most likely been subjected to some form of signal degradation such as intersymbol interference caused by multipath propagation, is demodulated by applying the received signal to a coherent phase detector 16. As an example, the coherent phase detector comprises a limiter-discriminator, which recovers the phase modulation component of the form d.PHI..sub.m t/dt, followed by an integrator from which the original M-ary digital data is derived.
In the demodulation/data recovery scheme described in the above-referenced patent, the integration function is accomplished by coupling the output of a limiter-discriminator through an aperture filter 17 to an accumulator 18. Each of the aperture filters associated with the transmit voltage-controlled oscillator and that coupled with the data recovery accumulator has a finite baseband time duration T corresponding to the inverse of the pulse repetition rate of the digital data. The accumulator sums successive samples (sampled every T seconds to derive data samples at T, 2T, 3T, . . . , NT seconds) in order to obtain an output representative of the accumulated phase of the transmitted RF signal and thereby permit recovery of the original M-ary digital data.
Now, although the communication scheme described in the above referenced patent provides an improved mechanism for digital data communication through continuous phase modulation, the spectrum of signals produced as a result of the use of a finite impulse response aperture filter mechanism (at the input to the modulator's VCO) and radiated by the transmitter contains sidelobes of substantial magnitude (adjacent to a mainlobe at the transmitted center frequency) which limit the extent to which multiple information channels may be stacked together within the narrowband communication channel without being subjected to unwanted mutual interference.